Surface-grinding machine

ABSTRACT

The invention contemplates surface-grinding of a workpiece wherein the surface to be ground is not necessarily flat and wherein a ground surface smoothness is produced with minimum removal of work material; in other words, the adequately smooth ground surface will reflect to some extent any major undulation in the unground surface. To the achievement of this result the grinding tool is fluid-pressure fed to and loaded upon the work surface in such manner as to &#39;&#39;&#39;&#39;float&#39;&#39;&#39;&#39; upon or self-adapt to profile change in the workpiece. In a horizontal surface grinding embodiment of the machine, a fluid-pressure operated counterbalance effectively offsets the weight of the grinding tool, its drive and its mounting, so that tool-positioning and work-loading portions of the system may function with minimum adverse inertial lag and/or mechanical hysteresis.

United States Patent Bain Aug. 5, 1975 SURFACE-GRINDING MACHINE [57] ABSTRACT [75] Inventor: James Douglas Bain, Akron, NY. The invention contemplates surface-grinding of a workpiece wherein the surface to be ground is not [73] Asslgnee' Amax New York necessarily flat and wherein a ground surface smooth- [22] Filed: May 24, 1974 ness is produced with minimum removal of work ma terial; in other words. the adequately smooth ground [2]] Appl' 473l78 surface will reflect to some extent any major undulation in the unground surface. To the achievement of [52] U.S. Cl 51/56; 5 Ill 10 this result the grinding tool is fluid-pressure fed to and [SI] Int. Cl B24b 7/00; B24b 9/00 loaded upon the work surface in such manner as to [58] Field of Search 51/54. 56. 110, 109 R float" upon or self-adapt to profile change in the 5l/340 workpiece. In a horizontal surface grinding embodiment of the machine, a fluid-pressure operated coun- [56] References Cited terbalance effectively offsets the weight of the grindrr STATES PATENTS ing tool, its drive and its mounting, so that tool- 1 773.3ss 8/1930 Canning 5. 51/54 Pmmoning, "f offhe 2874517 2/1959 Markle 51/56 may function with mmlrnum adverse inertial lag and- 2.881569 4/1959 Strnad et =11 51/56 /of mechanical hysteresis $751,855 8/1973 Pinat 51/56 Primary Examiner0thell Ml Simpson Attorney, Age/11, 0r FirmKasper T, Serijan; Roy C.

Hopgood 13 Claims, 4 Drawing Figures PATENTEU RUB 51975 SHEET SURFACE-GRINDING MACHINE The in lllOIl relates to suil'ttcogiintling ill a workpiece such as a metal slab, as for example to produce enough surface uniformity in opposite surfaces to enable ultrasonic exploration for internal defects in the workpiece.

In the production of certain metal alloys, such as zir conium alloys to be used in reactors and other equipment for the nuclear-power and chemical industries, it is essential to subject forged slabs of such alloy to thorough ultrasonic inspection before subjecting the slab to the further expensive processing needed to complete fabrication. In order to perform a meaningful ultrasonic test on a forged zircaloy slab, for example, a piece weighing 2600 lbs. and having substantially the dimensions 4-in. by 18-in. and l4-ft. long, both the jamor surfaces (IS-in. by 14-ft.) must be finished to what is generally known as 120 R.M.S., meaning 120 micro-inch deviation from the local neutral plane of the surface. In the past, the philosophy for achieving such surface quality has been that a truly flat plane must be achieved; this was done by first subjecting an abrasive blasted (i.e., oxide-free) forged slab to a swingframe grinder operation, using a 3-in. wide, l8-in. diameter wheel, under manual control by a skilled and patient operator, and by then finish-grinding with a conventional surface grinder, to hopefully remove all the small ridges (with peaks and valleys) produced in swing grinding. Not only were surface-rejection rates high, but there was an undue consumption of time and/or wastage of slab material in achieving the necessary quality of surface finish. Since rejections came as a result of the ultrasonic testing, slabs had to be reworked several times, resulting in excessive expenditure of manpower for regrinding and retesting, plus increased metal-conditioning loss. The excessive conditioning time created a severe bottleneck in production flow, with attendant loss of plant capacity.

It is, accordingly, an object of the invention to provide a surface-grinding machine and technique to avoid or substantially reduce past difficulties, of the above noted nature.

It is a specific object to provide a single surfacegrinding machine to condition workpiece surfaces for acceptability in ultrasonic testing of the character indicated, with minimum wastage through removal of workpiece material.

It is another specific object to achieve the above objects while eliminating the need for swing frame grinding.

Another specific object is to achieve surface quality in the order of 120 R.M.S. in a single grinding set up of each of the opposite major surfaces of a forged workpiece, which may have a twisted or undulatory profile, without removing the quantity of stock necessary to eliminate such a profile, i.e., without removing material to the lowest point of defect.

A further specific object is to achieve the foregoing objects with a surface grinder having a floating" capability, namely, self-adaptation to the primary surface profile of the workpiece.

It is a general object to achieve the foregoing objects with relative simplicity, economy, reliability, safety, and inherently high productivity.

Other objects and various further features of novelty and invention will be pointed out or will occur to those skilled in the art from a reading of the following specifi' cation, in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, a preferred form of the invention:

FIG. 1 is a simplified perspective view of my machine;

FIG. 2 is a plan view of slide-suspension structure for the grinding head of the machine of FIG. 1;

FIG. 3 is a front elevation of mechanism of FIG. 2, as seen at a section taken along the line 33 of FIG. 2; and

FIG. 4 is a simplified diagram of control elements for the machine.

Referring initially to FIG. 1, the invention is shown in application to a machine employing a flat wheel" 10 to grind an upwardly facing major surface of an elongate workpiece ll, securely held by plural edge clamps 12 to the sides of an elongate work-supporting table or pan 13. Table 13 is elevated by spaced fourwheel trucks l4 riding a floor-mounted frame of guide rails 15 which extends through and beyond both sides of floor-mounted support structure for the grinding wheel 10. Horizontal-axis pinned connection, as at [6, enables a degree of articulated adaptation of table and workpiece orientation to minor undulation or other imperfections in guide rails 15. Traverse drive means for the workpiece table is suggested by a motor 17 in one of the trucks l4 and having appropriate reduction-gear or other mechanical connection 18 to wheels of the truckv The flat wheel 10 is preferably a cylindrical chuck of the variety identified with Blanchard type grinders, wherein plural abrasive segments are clamped in angularly spaced relation, projecting axially downward, for abrasive working of the exposed horizontal surface of the workpiece. In the case of a forged workpiece slab ll of the dimensional magnitudes indicated, I use such a Blanchard flat wheel 10 wherein the effective work ing diameter of the projecting abrasive segments is 12 inches, and the wheel 10 is supported by the rotor shaft ofa drive motor 19; motor 19 may be of 25 hp. rating, providing a drive speed of 1,750 r.p.m., thus generating an effective surface-cutting speed of substantially 5,500 ft/min.

As will later more clearly appear, the grinding head, i.e., wheel 10 and its drive motor 19, are carried by a slide vertically guided and positionable within a boxlike cross-slide 20, and bosses 21 on the sides of crossslide 20 slidably support the same on fixed horizontal guide bars 22. The work-station frame comprises front and rear stands, connected by cross members 23 and protective panels 24. At the front stand, two columns 25 rise from a floor-mounting plate 26, and columns 25 are interconnected by a front plate 27 to which front ends of the horizontal guide bars 22 are secured; also at the front stand, a bracket 28, secured to plate 27 and with floor support via a leg 29, mounts double-acting actuator means 30 for positioning cross-slide 20 along its guide bars 22. At the rear stand, two columns 31 rise from a floor plate 32 and are interconnected by a rear plate 33 to which rear ends of the horizontal guide bars are secured. Upper and lower fluid-pressure operated devices 34-35 are mounted to upper and lower plates of the crossslide 20, for vertical-positioning control of the grinding head. Finally, a suitable enclosure 36 for exhausting and collecting products of abrasive action stands at the rear of the work station.

1n FIGS. 2 and 3, many of the described parts will be recognized. Additionally, the horizontal guide bars 22 for the cross-slide 20 are seen to be stoutly rooted to plates 27-33 by flanged bosses 37, and further front and rear longitudinal members 3839 reinforce the frame. In similar fashion, flanged bosses 40 firmly secure ends of vertical-slide guide bars 41 to the upper and lower plates of cross-slide 20, and the vertical slide is seen to comprise a plate 42, mounting bosses 43 guided by bars 41 and also mounting the motor 19. A somewhat trapezoidal extension 44 of the cross-slide 20 encompasses motor 19 and provides the means whereby actuating connection is made to the crossslide from the positioning rod 45 of actuator 30, as at a pinned clevis 46. Operative connection of the crossslide-mounted fluid-pressure devices 34-35, to the vertical slide 42, is best seen in FIG. 4, wherein the piston rod of each has separate pinned connection to brackets 47 which will be understood to be fixed to the rear face of slide plate 42, between locations of bosses 43 thereon.

It is a feature of the invention that the vertical position of the grinding head (i.e., vertical slide 42 and all that it carries) shall be able to self-adapt to the local profile elevation of the workpiece, all while maintain ing substantially the same tool-feeding axial-force load upon the workpiece. To this end, 1 provide counterbalance means whereby the full effective weight of slide 42 (and what it carries) is offset, so that axial toolloading force can be truly the sole job of the actuating means 34. Such counterbalance could be provided by a suitable system of pulleys and counterweights carried by cross'slide 20, but 1 indicate my preference for such compensation via fluid-pressure operated means 35. As shown, a regulator valve 50 runs from the main air supply of the plant and has an output connection 51 to the head end of the fluid-pressure operated means 35, the tail end of which exhausts to atmosphere, preferably via suitable bleed means, indicated by legend. An accumulator S2 is connected to line 51 at or close to the head end of means 35, and a throttle valve 53 provides a convenient means of selecting a steady rate of venting to atmosphere. Valves 50-53 are adjusted to fully off set the weight of slide 42 and the components which it carries.

As the tool-feeding actuator 34, my preference is to use an air-loaded hydraulic system of control. Such system is shown to comprise first and second accumulator tanks 54-55 which respectively serve the head and tail port connections to actuator 34. These tanks are partly filled with hydraulic fluid such as oil, which is the control medium supplying all ensuing operative connec' tions to actuator 34; remaining tank volume is airfilled, under pressure.

Specifically, the oil-outlet connection from tank 54 to the head port of actuator 34 is shown to include a first branch containing a throttle valve 56 and a shunt or second branch containing a check valve 57', valve 57 is directionally oriented to pass only for flow which exhausts the head end of actuator 34, and the throttle valve 56 is adjusted for desired slowly metered downward feed displacement of the tool 10. In similar fashion, the oil-outlet connection from tank 55 to the tail port of actuator 34 includes a throttle valve 58, preferably adjusted in relation to the adjustment of valve 56 so as to provide for such greater flow (as compared to the setting at 56) as to produce a relatively rapid toolretracting displacement rate. A stop valve 59 provides a means for selectively interconnecting the oil outlets of tanks 54-55, as when needed to adjust the degree to which the respective tanks are oil-filled.

Accumulators 54-55 are air-loaded at reduced pressure, derived from the main air supply by a regulating valve 60, depending upon the instantaneous setting of a Feed-Retract" selector valve 61; as shown, valve 61 has been upwardly actuated to its tool Feed" position,

thus (a) connecting regulated air pressure to the toolfeed accumulator 54 and (b) connecting the air volume of accumulator 55 for atmospheric venting. 1n the toolretracting or down position of valve 61, connections are reversed, and the regulated control air is supplied to the tool-retracting accumulator 55, while the air volume in accumulator 54 is connected for atmospheric venting. A relief valve 62 is shown in the regulated-air line to valve 61, the same being set to vent at the predetermined regulated pressure, as an assurance against ever applying greater than desired air-pressure to the accumulator system.

The remainder of the control system, i.e., for table motor 17 and for cross-slide actuator 30, may be of conventional form and may be operated from the same main air supply. It suffices here to identify cross-feed control means 63 with its own regulator-valve connection 64 to the air supply and also to identify workfeed control means 65 similarly served by a separate regulating-valve connection 66 to the main air. Legends IN and OUT applied to a manual actuator will be understood to designate selective in-out actuating displacement of the cross-slide 20 via means 30, a given actuated displacement being retained for a midposition setting of such actuator. Similarly, legends R-L and L-R" applied to a manual actuator will be understood to designate selective left-to-right or right-to-left actuating displacement of the work-supporting table 13, a given actuated displacement (e.g., end of a work traverse) being retained for a mid-position setting of such actuator. All control will be understood to be available to the operator at a single console location (not shown), preferably affording him a safe yet direct view of the region of tool-to-work contact, and providing such additional monitoring information as a meter display of motor-current consumption.

In use, a forged slab 11 is suitably shimmed, leveled and clamped at 12 in position on table 13, the slab having previously been abrasive-blasted to remove surface oxide. The cross-slide feed means 63 is adjusted to have the tool cut a generous swath during a longitudinal traverse stroke of the table; for a 12-in. diameter abrasive wheel 10, this generally means a swath 11 inches wide, Le, a 1-inch projection of the tool swath laterally outside a longitudinal side edge of the workpiece. The work-feed control means 65 is adjusted to permit tool 10 to be lowered into substantial-area contact with one end of the workpiece. Motor 19 is then started, and control valve 61 set for tool-feed connections; adjust- 60 ment at 56 for a tool-feed rate of about 5 inches per minute is illustrative and satisfactory. Upon tool contact with the workpiece. work-feed control means 65 is activated to determine the appropriate direction of longitudinal traverse, a selected rate of 60 to 70 65 inches per minute being illustrative and satisfactory. Upon tool contact with the workpiece, a downward tool-feeding force will encounter an opposing work reaction, and a force-limited condition will be established, based on the regulated pressure delivered by valve 60 (the vent setting of relief valve 62) and the effective area of the piston element of fluid-pressure op erated means 34. In the course of the traverse stroke, a local drop in elevation of the workpiece surface will be accommodated by an incremental feeding descent of the tool, in that the feed-loading force of the tool against the work remains constant; similarly, a local rise in elevation of the workpiece surface will be signaled by upward displacement of the piston at 34, with accompanying rise in liquid level in accumulator 54 and such venting of air alias is necessary. Thus, at all times the feeding force behind abrasive action remains substantially constant, allowing the grinding head to float in response to work reaction to the cut.

At the end of the first longitudinal traverse stroke, say, right-to-left along a front-edge swath as already described, the operator (or, if desired, preset limit switches, not shown) terminates work feed at 65 and initiates at 63 a short IN stroke of the cross-slide 20. This end of the longitudinal stroke occurs when the tool begins to run off the left end of the workpiece and well before tool contact area with the work has reduced to 75 percent. The short cross-slide displacement places the tool in approximately l-inch overhanging relation with the other longitudinal edge, so that for an assumed 18-inch width of the workpiece, the crossfeed stroke is about 8 inches. The return longitudinal stroke (left-to-right) is then initiated, and is followed by a return cross-feed stroke, to complete a cutting cycle.

Ordinarily, the work-loading pressure settings at 60 and at 62 are such that the motor 19 runs at about 90 percent of its maximum load rating; this can be visually monitored by the display of motor current consumption. For the indicated illustrative dimensions and upon a zircaloy workpiece, this means removal of approximately 0.010 inch of workpiece material from the surface being ground. Of course, when the first major surface of the workpiece appears to have the requisite smoothness, it is remounted upside-down, so that the opposite major surface can be similarly ground. Ordinarily, satisfactory smoothness is achieved for a 2,600- pound zircaloy slab of the character indicated, upon removal of 60 to 70 lbs. of slab surface. To date, in my experience, this represents a remarkably low figure, achieved by not insisting that the ground surfaces be flat.

Even though the surfaces ground by my machine will appear to be scored, this is not such a scoring as to exceed the indicated 120 R.M.S. limitation. It is customary to then subject the surfaces to a brief exposure to centrifugal shot-peening with steel-grit abrasive to develop a matte"-like finish, prior to immersion in the water tank used for ultrasonic inspection. It is my experience that rejection rates, at the ultrasonic-inspection stage, have been reduced to substantially zero, for workpieces ground in my machine and processed as indicated; such rejection as does occur is the result of a discovered internal defect and is no longer the result of such surface-ground anomaly as will impair the detecting efficacy of the ultrasonic apparatus. As a result, my machine has achieved tremendous savings in manpower, production efficiency, and percent utilization of originally forged stock material.

The described invention will be seen to have met all stated objects. And it will be appreciated that modifications may be made without departing from the scope of the invention as defined in the claims. For example, in the event that the piston-area at 34 is small compared to the area of the air-oil interface at 54, the volume displacement at 34 accompanying tool-position adaptation to a local rise in work-surface elevation will be a very small fraction of the air volume at 54; this fact, coupled with the inherent compressibility of air, will means graceful accommodation of the necessary and automatic repositioning of the tool with negligible rise in air pressure at 54, thus obviating any strict need for the relief valve 62. Thus, for practical purposes, relief valve 62 is not needed for floating action, but it may be regarded as a back-up safety measure to assure against any tool-loaded contact with the workpiece beyond the preselected level of downward force.

What is claimed is:

1. A surface grinding machine comprising a workstation frame, horizontally oriented work-supporting means and means for horizontally moving the same with respect to said work-station frame, vertical slide and guide means positioned by said frame above the path of movement of said work-supporting means, toolholding means, rotary drive means on said slide means for rotary mounting and drive of said tool-holding means on a vertical axis and in downwardly facing orientation, counter-balance means for said slide means to offset the combined weight of said slide means and of the tool and drive elements carried thereby, and fluidpressure operated means including a tool-feeding stroke for vertical positioning of the tool in respect of work mounted in said work-supporting means; said fluid-pressure operated means in its tool-feeding stroke connecton including a regulated air supply to an accumulator, a throttle-valve connection of said accumulator to said fluid-pressure operated means, and checkvalve means connected in shunt across said throttlevalve connection, said check-valve means blocking fluid flow to said fluid-pressure operated means and passing flow from said fluid-pressure operated means; the volume displacement within said fluid-pressure operated means, in reaction to surface undulation of the work, being so small compared to the available air volume in said accumulator, that the tool engages the work with substantially constant force for a range of surface undulations of the work.

2. The machine of claim 1, in which said worksupporting means is horizontally elongate, and in which the movement thereof is longitudinal.

3. The machine of claim 1, in which said fluidpressure operated means is double-acting and has a tool-retracting stroke for which the control connection includes a throttle valve between the air supply and said fluid-pressure operated means.

4. The machine of claim 3, in which a second accumulator is in the air-supply connection to said lastmentioned throttle valve.

5. The machine of claim 1, in which the framepositioning reference for said vertical slide and guide means is a horizontal slide and frame-based horizontal guide means therefor, said horizontal guide means being generally transverse to the path of work movement provided by said horizontally moving means.

6. The mechanism of claim 1, in which said counterbalance means is fluid-pressure operated, and selectively operable pressure-regulator means for controlling the pressure of fluid supplied to said fluid-pressure operated means.

7. The mechanism of claim 6, in which a pressure fluid accumulator is connected to said fluidpressure operated means.

8. The machine of claim 1, in which said accumulator is partially filled with liquid and in which said fluidpressure operated means and the accumulator connection thereto is also liquidfilled.

9. A surface-grinding machine comprising a work station frame, horizontally oriented work-supporting means and means for horizontally moving the same with respect to said work-station frame, vertical slide and guide means positioned by said frame above the path of movement of said work-supporting means, toolholding means, rotary drive means on said slide means for rotary mounting and drive of said tool-holding means on a vertical axis and in downwardly facing orientation, counter-balance means for said slide means to tors are part-air and part-liquid filled, and in which the liquid filling extends from said accumulators continuously to and fills said fluid-pressure operated means.

ll. A surface-grinding machine comprising a workstation frame, horizontally oriented work-supporting means and means for horizontally moving the same with respect to said work-station frame, vertical slide and guide means positioned by said frame above the path of movement of said work-supporting means, toolholding means, rotary drive means on said slide means for rotary mounting and drive of said tool-holding means on a vertical axis and in downwardly facing orientation, counter-balance means for said slide means to offset the combined weight of said slide means and of the tool and drive elements carried thereby, and fluidpressure operated means including a first pressure-fluid port for a tool-feeding stroke and a second pressurefiuid port for a tool-retracting stroke in the vertical positioning of the tool in respect of work mounted in said offset the combined weight of said slide means and of work-supporting means; said fluid-pressure operated the tool and drive elements carried thereby, and fluidpressure operated means including a first pressure-fluid port for a tool-feeding stroke and a second pressurefluid port for a tool-retracting stroke in the vertical positioning of the tool in respect of work mounted in said work supporting means; said fluid-pressure operated means for said stroke connections including a regulated air supply with a selectonvalve connection thereof to a selected one of two accumulators, a first throttlevalve connection of one of said accumulators to said first port, a second throttle-valve connection of the other of said accumulators to said second port; the volume displacement within said fluid-pressure operated means. in reaction to surface undulation of the work, being so small compared to the available air volume in said one accumulator. that the tool engages the work with substantially constant force for a range of surface undulations of the work.

10. The machine of claim 9, in which said accumulameans for said stroke connections including a pressurized air supply with a selectorvalve connection thereof to a selected one of two control outlets, a first throttlevalve connection of one control outlet to said first port, a second throttle-valve connection of the other control outlet to said second port, and relief-valve means connected to vent at least said first throttle-valve connection for fluid pressures in excess of a predetermined magnitude.

12. The machine of claim ll, in which at least said first throttle-valve connection includes an accumulator between said relief-valve connection and the throttlevalve part of said first throttle-valve connection.

13. The machine of claim 12, in which said accumulator is part-air and part-liquid filled, and in which the liquid filling extends from said accumulator continuously to and fills at least that part of said fluid-pressure operated means which is accessible via said first port k 

1. A surface grinding machine comprising a work-station frame, horizontally oriented work-supporting means and means for horizontally movinG the same with respect to said work-station frame, vertical slide and guide means positioned by said frame above the path of movement of said work-supporting means, toolholding means, rotary drive means on said slide means for rotary mounting and drive of said tool-holding means on a vertical axis and in downwardly facing orientation, counter-balance means for said slide means to offset the combined weight of said slide means and of the tool and drive elements carried thereby, and fluid-pressure operated means including a tool-feeding stroke for vertical positioning of the tool in respect of work mounted in said work-supporting means; said fluid-pressure operated means in its tool-feeding stroke connecton including a regulated air supply to an accumulator, a throttle-valve connection of said accumulator to said fluid-pressure operated means, and checkvalve means connected in shunt across said throttle-valve connection, said check-valve means blocking fluid flow to said fluid-pressure operated means and passing flow from said fluidpressure operated means; the volume displacement within said fluid-pressure operated means, in reaction to surface undulation of the work, being so small compared to the available air volume in said accumulator, that the tool engages the work with substantially constant force for a range of surface undulations of the work.
 2. The machine of claim 1, in which said work-supporting means is horizontally elongate, and in which the movement thereof is longitudinal.
 3. The machine of claim 1, in which said fluid-pressure operated means is double-acting and has a tool-retracting stroke for which the control connection includes a throttle valve between the air supply and said fluid-pressure operated means.
 4. The machine of claim 3, in which a second accumulator is in the air-supply connection to said last-mentioned throttle valve.
 5. The machine of claim 1, in which the frame-positioning reference for said vertical slide and guide means is a horizontal slide and frame-based horizontal guide means therefor, said horizontal guide means being generally transverse to the path of work movement provided by said horizontally moving means.
 6. The mechanism of claim 1, in which said counterbalance means is fluid-pressure operated, and selectively operable pressure-regulator means for controlling the pressure of fluid supplied to said fluid-pressure operated means.
 7. The mechanism of claim 6, in which a pressure-fluid accumulator is connected to said fluid-pressure operated means.
 8. The machine of claim 1, in which said accumulator is partially filled with liquid and in which said fluid-pressure operated means and the accumulator connection thereto is also liquid-filled.
 9. A surface-grinding machine comprising a work-station frame, horizontally oriented work-supporting means and means for horizontally moving the same with respect to said work-station frame, vertical slide and guide means positioned by said frame above the path of movement of said work-supporting means, tool-holding means, rotary drive means on said slide means for rotary mounting and drive of said tool-holding means on a vertical axis and in downwardly facing orientation, counter-balance means for said slide means to offset the combined weight of said slide means and of the tool and drive elements carried thereby, and fluid-pressure operated means including a first pressure-fluid port for a tool-feeding stroke and a second pressure-fluid port for a tool-retracting stroke in the vertical positioning of the tool in respect of work mounted in said work-supporting means; said fluid-pressure operated means for said stroke connections including a regulated air supply with a selector-valve connection thereof to a selected one of two accumulators, a first throttle-valve connection of one of said accumulators to said first port, a second throttle-valve connection of the other of said accumulators to said second port; the volume displacement within said fluid-pressure operated means, iN reaction to surface undulation of the work, being so small compared to the available air volume in said one accumulator, that the tool engages the work with substantially constant force for a range of surface undulations of the work.
 10. The machine of claim 9, in which said accumulators are part-air and part-liquid filled, and in which the liquid filling extends from said accumulators continuously to and fills said fluid-pressure operated means.
 11. A surface-grinding machine comprising a work-station frame, horizontally oriented work-supporting means and means for horizontally moving the same with respect to said work-station frame, vertical slide and guide means positioned by said frame above the path of movement of said work-supporting means, tool-holding means, rotary drive means on said slide means for rotary mounting and drive of said tool-holding means on a vertical axis and in downwardly facing orientation, counter-balance means for said slide means to offset the combined weight of said slide means and of the tool and drive elements carried thereby, and fluid-pressure operated means including a first pressure-fluid port for a tool-feeding stroke and a second pressure-fluid port for a tool-retracting stroke in the vertical positioning of the tool in respect of work mounted in said work-supporting means; said fluid-pressure operated means for said stroke connections including a pressurized air supply with a selector-valve connection thereof to a selected one of two control outlets, a first throttle-valve connection of one control outlet to said first port, a second throttle-valve connection of the other control outlet to said second port, and relief-valve means connected to vent at least said first throttle-valve connection for fluid pressures in excess of a predetermined magnitude.
 12. The machine of claim 11, in which at least said first throttle-valve connection includes an accumulator between said relief-valve connection and the throttle-valve part of said first throttle-valve connection.
 13. The machine of claim 12, in which said accumulator is part-air and part-liquid filled, and in which the liquid filling extends from said accumulator continuously to and fills at least that part of said fluid-pressure operated means which is accessible via said first port. 